1. Field of the Invention
This invention relates to a radio frequency identification (RFID) system that limits propagation of radio signals. In particular, the radio signals are diminished outside of an area monitored by the RFID system
2. Description of the Related Art
A radio frequency identification (RFID) system is useful for many applications. One application involves tracking items in a business. The RFID system can receive data from transponders (referred to as “tags”) affixed to the items. By “reading” the data, the RFID system can be used by the business to identify each item with a tag and associated location.
The RFID system generally includes an antenna, a reader, a server, an operator terminal and a database. The antenna transmits a radio signal that is received from the reader. The radio signal is received by a tag (RFID transponder) affixed to an item. The tag, in turn, responds to the radio signal by sending a response radio signal that includes data stored within the tag. The reader receives the responding radio signals via the antenna. From the responding radio signal, the reader can extract the data sent by the tag. The data is generally sent to the server from which the data can be accessed by the operator terminal or stored in the database.
The RFID tag or transponder can be one of passive, semi-passive (also referred to as “battery-assisted”), and active. A passive RFID tag does not have a battery or a transmitter. A passive tag receives energy to operate from the radio signal transmitted by the RFID reader. The passive tag transmits a response to the radio signal by modulating and reflecting the radio signal. One example of the passive tag is the tag contained within an access control badge such as the tags manufactured by HID Global of Irvine, Calif. A semi-passive tag (or semi-active) tag includes a battery to supply energy for operation. The semi-passive tag does not include a transmitter. The semi-passive tag also operates by modulating and reflecting the radio signal. One example of the semi-passive tag is the E-ZPass tag used for electronic toll collection. An active tag includes a battery and a transmitter. Unlike the passive and semi-passive tags, the active tag is capable of initiating a transmission to the reader. One example of the active tag is the tag used for object location produced by WhereNet of Santa Clara, Calif.
In one application of the RFID system, the RFID system is used to identify tagged items as the items pass through a receiving portal. In one embodiment, the reader is turned on in advance of the tagged item entering a receiving portal. The reader is turned on in advance to allow sufficient time to read the tag. After the tag has been read and passed through the portal, the reader is turned off to save on the duty cycle of the reader.
A portal may separate regions or areas while still allowing for the passage of physical objects such as people, packages, and machinery. Some examples of portals include a gateway separating a loading dock from a receiving area, an opening between two divided areas in a retail establishment, an opening through which luggage is transported on a conveyer belt in an airport, and spaces or openings through which packages pass in a sorting facility at a delivery company.
In some applications, after a tagged item has passed through the portal, the item is stored in a staging area nearby the portal. In some instances, the reader may read a tag in the nearby staging area. It is not desired to read the same tag again in the staging area after the tag was already read while passing through the portal.
Reading the same tag multiple times can lead to problems such as misinterpretation by a business computing system. For example, reading a tag on an item that was already received through the portal may result in an erroneous message being communicated to an operator that an unexpected item is being received.
Another problem with the reader reading a tag in the nearby staging area is that it may be difficult to determine the location of the tag. In one application, multiple readers may be used with each reader assigned to a separate area. If readers in adjacent areas read the same tag, an operator may not be able to determine which area the tag is in. Additionally, when adjacent areas are each equipped with a reader, the readers may interfere with each other. The interference may occur because power levels of signals emitted from antennas associated with the readers are many orders of magnitude greater than the response radio signals emitted from the tags.
Attempts have been made to avoid a reader reading tags in nearby areas. One attempt includes restricting the power levels of radio signals emitted by the reader. Restricting the power levels limits the range of the radio signals. However, it has been found that restricting the power levels also decreases the accuracy of reading the tags.
Therefore, what are needed are techniques that limit a signal emitted from an antenna associated with a reader from propagating to an unwanted area. In particular, the techniques do not reduce the accuracy of reading a tag.